As anyone who knows about tube amps will tell you, “The output transformer is the most expensive part”. Not is it is only the most expensive part, but is is also probably the hardest part to come by. Parts for tube amps in general are a lot harder to find then other more common electronic [...]

What is a Vacuum Tube ?
In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. For most purposes, the vacuum tube has been replaced by the much [...]

This is such an in depth subject that I will reserve my input to this component to more basic explanations and leave the permeability and make-up of core material, magnetic flux density, shunt capacitance/inductance, electromagnetic theory and the like
to references.

A SE transformer has to do two things in life… carry the unbalanced DC and have enough magnetic oomph to do the AC duties well. PP outputs (largely so) are relieved of the DC duties. As a very general approximation the SE transformer will be four times as large as the PP transformer for equivalent [...]